1. Field of the Invention
The present invention relates to a magneto-optical recording method capable of performing an overwrite operation, and a magneto-optical recording medium for use in the above magneto-optical recording method.
2. Discussion of Background
Since laser beams are employed when information is recorded in or reproduced from a magneto-optical recording medium, as in conventional optical recording media, not only an enormous volume of information can be recorded, but also overwriting is possible. In addition to the above, recording and reproduction of information can be performed with a magnetic recording head being out of contact with the recording medium, so that the recording medium is protected from dusts. The magneto-optical recording medium is thus quite stable in recording and reproduction performance.
Studies are now being made on such magneto-optical recording media by placing great emphasis on its utilization for filing documents, and video and motionless pictures; and for computer memories. Substitution of the magneto-optical recording medium for a floppy disk or hard disk is also highly expected, and commercialization thereof will be achieved in the near future.
A variety of magnetic films are proposed for a recording layer of the above-mentioned magneto-optical recording medium. These magnetic films are made of amorphous magnetic alloys such as TbFe, TbFeCo, GdTbFeCo and NdDyFeCo, using transition metals such as Fe and Co and rare earth metals such as Gd, Dy, Tb and Nd in combination. In the magneto-optical recording media using the above-mentioned magnetic alloy layers, noise as caused by boundaries of crystals in the media is minimal by virtue of the amorphous state thereof. Furthermore, the above amorphous magnetic alloy layers can be easily formed on a substrate by sputtering or vacuum-deposition in the form of a vertically magnetized layer.
As the recent progress of the recording speed and density in the field of information recording, a demand for a magneto-optical recording medium which has further improved magneto-optical characteristics such as Curie temperature (Tc), coercive force (Hc) and Kerr rotation angle (.theta.k). To obtain the improved magneto-optical recording medium, materials for the magneto-optical recording medium have been developed and the recording layers of the recording medium have been improved. For instance, there are proposed function-separating type magneto-optical recording media having a two-layered recording layer in Japanese Laid-Open Patent Applications 56-153546, 57-78652, 60-177455 and 63-153752. More specifically, this function-separating type magneto-optical recording medium comprises two recording layers, which are separately capable of recording and reproducing information.
In the magneto-optical recording medium, a light modulation method is generally employed for recording and erasing information. In the light modulation method, information signals are modulated by laser beams, and a bias magnetic field is applied to the recording medium, with its direction changed, depending on the recording and erasing operations.
Unlike the recording method employed in a hard disk, the above-mentioned light modulation method is not capable of overwriting information. To record some information in an area where other information has been already recorded therein, two steps, that is, first, an erasing operation with respect to the previously recorded information and then a recording operation of new information, are required. In this recording method, the disadvantageous prolongation of the access time is inevitable.
To solve the above-mentioned problem, the applicant of the present application has proposed a magnetic field modulation method, as disclosed in Japanese Laid-Open Patent Application 57-113402. According to this magnetic field modulation method, an information signal is modulated by a magnetic field generated by a magnetic head, and provided to a magneto-optical recording medium, with laser beams continuously applied thereto. This magnetic field modulation method is capable of overwriting information, just like the recording method as employed in a hard disk.
From the viewpoint of substitution of the magneto-optical recording medium for a hard disk, the magneto-optical recording medium is required to record information as quickly as, or more quickly than the hard disk, for example, at a disk rotation speed of 3600 rpm and a linear velocity of 22 m/sec or more.
To record information at such a high speed, however, there are the following drawbacks in the magneto-optical recording medium employing the above-mentioned magnetic field modulation method:
1. It will be necessary to reverse the direction of the magnetic field generated by the magnetic head at a frequency as high as 15 MHz or more in accordance with the recording and erasing operations. This is considered to be practically difficult. PA1 2. To reverse the magnetization direction of the vertically magnetized recording layer of the magneto-optical recording medium, a magnetic field with an intensity of as great as 200 to 300 Oe is required. In order to obtain the magnetic field with this intensity, the magnetic head must be floated so as to be almost in contact with the magneto-optical recording medium, which produces problems of wear and damage to both the magnetic head and the magneto-optical recording medium, and impairs the reliability of this magneto-optical recording method. PA1 3. When information is recorded at high frequency, recording bits are affected by the heat conduction and thermal magnetic recording characteristics of the magneto-optical recording medium and thus shaped like a crescent-form leaving a trail. This causes noise in the course of reproduction and degrades the C/N ratio.
To eliminate the above-mentioned drawbacks, there is proposed a recording method capable of performing the overwrite operation by changing the light intensity of a single laser beam, as shown in Japanese Laid-Open Patent Application 62-175948 and in National Convention Record p.721,28P-ZL-3 of The Institute of Applied Physics held in 1987.
According to the magneto-optical recording method described in the above references, a magneto-optical recording medium which comprises a two-layered, ferrimagnetic coupling type recording layer consisting of a memory layer and an auxiliary layer is used. This method employs the light intensity modulation method and necessitates an auxiliary magnet for initializing the recording medium. The process of the overwrite operation according to this magneto-optical recording method will be now explained in detail by referring to FIGS. 18 and 19.
The magneto-optical recording medium used in this method comprises a memory layer 31 and an auxiliary layer 32 as shown in FIG. 18(a). The memory layer 31 is made of an amorphous magnetic alloy, such as TbFeCo, comprising transition metals and rare earth metals in combination. In the memory layer 31, the magnetic moment of the employed transition metals is predominant over that of the rare earth metals at room temperature, the coercive force (Hc) is great, and the Curie Temperature (Tc) is low. On the other hand, the auxiliary layer 32 is made of an amorphous magnetic alloy, such as TbDyFeCo, comprising transition metals and rare earth metals in combination. In the auxiliary layer 32, the magnetic moment of the employed rare earth metals is predominant over that of the transition metals at room temperature, the coercive force (Hc) is small, and the Curie Temperature (Tc) is high. In FIG. 18(a), a.sub.1 indicates a recorded area where information is recorded therein and a.sub.2 indicates an erased area where information is erased therefrom. The thermal magnetic characteristics of each amorphous magnetic alloy layer of the memory layer 31 and the auxiliary layer 32 are shown in FIG. 19. In FIG. 19, Hex indicates a bias magnetic field, and T.sub.1 and T.sub.3 indicate the operation temperatures for erasing and recording information, respectively.
An initializing magnetic field Hin of 6 to 7 KOe is applied by an auxiliary magnet for initialization to a recording medium in which information has been recorded and erased as shown in FIG. 18(a) by a laser-beam-light-intensity modulation method. Since the portion of the auxiliary layer 32 in the recorded area a.sub.1 has a low coercive force at room temperature, only the magnetization direction of the portion of the auxiliary layer 32 in the recorded area a.sub.1 is reversed in the direction of the magnetic field for initialization as shown in FIG. 18(b). When a bias magnetic field Hex is applied to the recording medium in an upward vertical direction in the above-mentioned state, and a laser beam having a small power is applied thereto to increase the temperature thereof to a temperature (T.sub.1), the coercive force of the portion of the memory layer 31 in the recorded area a.sub.1 is drastically decreased as shown in FIG. 18(c). As a result, the magnetized direction of the portion of the memory layer 31 in the recorded area a.sub.1 is reversed by the exchange bonding action with the auxiliary layer 31. When the temperature is decreased in such a state, the recorded information in the recorded area a.sub.1 is erased as shown in FIG. (d). Starting from the state as shown in FIG. 18(d), when a laser beam having a large power is applied and the temperature of the auxiliary layer 32 is increased to a temperature T.sub.3 which is higher than the compensation temperature Tcomp thereof, the magnetization of the memory layer 31 is lost. Since the coercive force Hc of the auxiliary layer 32 is smaller than the intensity of the bias magnetic field Hex, the magnetization direction of the auxiliary layer 32 is reversed in the direction of the bias magnetic field Hex as shown in FIG. 18(e). When the temperature is lowered to room temperature in such a state, recording can be done as shown in FIG. 18(f).
However, the above-mentioned magneto-optical recording method includes the step of initializing the magnetization direction in the memory layer 31 and the auxiliary layer 32 by applying a magnetic field thereto with an intensity as great as about 6 kOe, prior to the recording and erasing operations. This initializing step requires a permanent magnet or an electromagnet, which makes it difficult to minimize the size of a magneto-optical head. Furthermore, the process of recording and erasing information is so complicated that it is considered to be difficult to put this magneto-optical recording method in practice.